You are correct sir, it's from the DLC, Artorias of the Abyss, I think. Plot being spoiled in the eye, so be aware. Manus, father of the abyss, pulls your character through time, back to when all of Gwyn's knights where still around. Artorias had been charged with defeating Manus and stopping the spread of the abyss, but failed. His last sane act was to use his greatshield to create a barrier to protect Sif, which you can see in the picture up above. He becomes corrupted by the abyss, and it falls to you to put him down. Afterwords, you do everything Artorias became famous for in his stead, saving Sif, as well as the local princess, defeating Manus and preventing the spread of the abyss, and defeating the last of the scaled dragons, all in Artorias' name. Also, if you do the DLC before fighting Sif, the cut-scene at the start of the fight changes. He recognizes you, but still refuses to give you the ring, because he's afraid you'll be corrupted by the abyss like Artorias was. He tries to kill you because he knows that death isn't permanent for you, and he hopes to save your sanity from the abyss.
Dark Souls is deep, man.